1. Field of the Invention
The present invention relates to magnetic powder and an isotropic bonded magnet produced using the magnetic powder.
2. Description of the Prior Art
For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetic performance (that is, magnetization) of the magnetic powder and the content (that is, compositional ratio) of the magnetic powder in the bonded magnet. Accordingly, when the magnetic performance (magnetization) of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.
At present, most of practically used high performance rare-earth bonded magnets use the isotropic bonded magnets which are made using MQP-B powder manufactured by MQI Corp. as the rare-earth magnetic powder thereof. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets such as those manufactured using MQP-B powder have the following disadvantages.    (1) The conventional isotropic bonded magnets do not have sufficiently high magnetic flux density. Specifically, because of the low magnetic performance (that is, the insufficient magnetization) of the magnetic powder used, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.    (2) Since the conventional bonded magnet has high coercivity (coercive force), magnetizability thereof is poor, thus requiring a relatively high magnetic field for magnetization.    (3) Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable for the practical motors are very low. Further, these magnets have poor heat stability due to their small coercive forces.    (4) The conventional bonded magnets have low temperature characteristics (that, is heat resisting property or heat stability). In particular, the irreversible flux loss remarkably drops when the coercive force (HCJ) is lowered.    (5) The bonded magnet has poor corrosion resistance and heat resisting property. In particular, when the content of the magnetic is increased in order to compensate the low magnetic performance of the magnetic powder (that is, when the magnetic flux density of the bonded magnet is extremely raised), the corrosion resistance and heat resisting property remarkably drop.
Therefore, it is necessary to cover the outer surface of the bonded magnet with a coating, especially a resin coating which is capable of obtaining high corrosion resistance property, but this in turn leads to increased manufacturing cost and the presence of the resin layer results in lowered magnetic performance (this makes it difficult for a motor to generate a high torque).